The present invention relates to a two-stroke cycle engine, and, more particularly, to a two-stroke cycle engine adapted for use with automobiles.
A two-stroke cycle engine has theoretically the advantage that an engine of a certain size can generate a greater power than a four-stroke cycle engine of a bigger size because the two-stroke cycle engine has twice as many work cycles per revolution as the four-stroke cycle engine. In fact, however, the conventional two-stroke cycle gasoline engine employing a carburetor has such drawbacks as that it has high fuel consumption as compared with the four-stroke cycle engine due to the loss of air-fuel mixture caused by the direct escape, i.e. blow-out, of scavenging mixture to the exhaust manifold during scavenging, and that it cannot generate such a high power as expected from the fact that is has twice as many work strokes as the corresponding four-stroke cycle engine, due to the fact that the scavenging is still insufficient. Because of these problems, the practical use of two-stroke cycle gasoline engines is presently limited to the field of small engines, which must be simple in structure and low in manufacturing cost.
Conventional two-stroke cycle gasoline engines of the abovementioned type, therefore, generally employ crankcase compression for scavenging. However, the scavenging by crankcase compression is not fully effective, and can only provide a relatively low volumetric efficiency. This is the principal cause of the poor output power of conventional two-stroke cycle gasoline engines. In fact, a volumetric efficiency as high as 80% is available in four-stroke cycle engines, while, on the other hand, the volumetric efficiency of typical two-stroke cycle engines is still as low as 40-50%. The pump stroke volume of crankcase compression is equal to the stroke volume of the engine. However, since the crankcase has a relatively large clearance volume, the compression ratio of crankcase compression is relatively low, so that as a result the amount of air-fuel mixture drawn into the crankcase is small, the amount of delivered mixture is small, the delivery pressure is low and hence the scavenging pressure is low, and consequently it is hard to supply a really adequate amount of scavenging mixture into the power cylinder. As a result, the delivery ratio obtained in an engine wherein scavenging is effected only by the normal crankcase compression is only as high as 0.5-0.8. Furthermore, since the trapping efficiency is about 0.7, the volumetric efficiency becomes as low as 40-50% as mentioned above.
The purpose of scavenging is to push the residual exhaust gases in the power cylinder out of it by fresh mixture, and, the therefore, if the pressure of the residual exhaust gases and the distance between the scavenging port and the exhaust port are given, the time required for completing scavenging is determined by the pressure and the amount of scavenging mixture, provided that stratified scavenging is performed. Now, if the scavenging pressure is low, as when crankcase compression is used, a relatively long time is required for completing scavenging, particularly when the scavenging is performed by uniflow scavenging. Therefore, when the engine is rotating at high speed, it may well occur that the exhaust port is closed before the scavenging is completed, so that a large amount of exhaust gas still remains in the power cylinder, and only a very small amount of fresh mixture is charged into the power cylinder. Therefore, conventional two-stroke cycle engines have been unable to operate satisfactorily in the high speed range.
In view of these problems, the idea of providing a special scavenging pump in addition to crankcases which are adapted to perform crankcase compression so as to increase the amount and the pressure of scavenging mixture so far that the volumetric efficiency be increased up to 75-90%, or in some cases even up to 100% has been proposed. This increases the output power of a two-stroke cycle gasoline engine per unit volume of its power cylinder, Lowering the rotational speed of such an engine depending upon the increase of output power per unit volume of the power cylinder permits scavenging with the increased amount and pressure of scavenging mixture without causing any substantial mixing between scavenging mixture and exhaust gases, and reduces power loss due to internal friction in the engine. Thus the output power per unit volume of the power cylinder or of the engine itself is even more increased. Further, reducing the volume, in particular the height of the engine, depending upon the aforesaid increased output power per unit volume of the engine, also reduces the height of the engine compartment, so that the air resistance of the vehicle is reduced, with corresponding improvement of the fuel consumption. We have proposed, in a co-pending U.S. patent application Ser. No. 917,244 now U.S. Pat. No. 4,287,859, a two-stroke cycle gasoline engine comprising at least one two-stroke cycle power cylinder - piston assembly incorporating uniflow scavenging and two horizontally opposed pistons, and a scavenging pump means including at least one pump cylinder - piston assembly of the reciprocating type driven by said power cylinder - piston assembly in synchronization therewith, wherein the total stroke volume of said scavenging pump means is between 1.35 and 1.85 times as large as that of said power cylinder - piston assembly, and the operational phase of a pump cylinder - piston assembly is so shifted relative to that of the power cylinder - piston assembly to which it supplies scavenging mixture that, when the power cylinder - piston assembly is at its bottom dead center, the pump cylinder - piston assembly is at or slightly before its top dead center.
In the abovementioned patent application, we have proposed, as an embodiment of the two-stroke cycle gasoline engine having the aforementioned basic structure, an engine which has two power cylinder - piston assemblies of the aforementioned type adapted to operate with a phase difference of 180.degree. therebetween and one double-acting reciprocating type pump cylinder - piston assembly incorporating two horizontally opposed pistons as the aforementioned pump cylinder - piston assembly, which is more compact as a whole and is able to generate large output power. This formerly proposed two-stroke cycle gasoline engine having a double-acting pump cylinder - piston assembly incorporating two horizontally opposed pistons includes a pair of common crankshafts adapted to rotate in synchronization with each other, wherein two two-stroke cycle power cylinder - piston assemblies each incorporating two horizontally opposed pistons have individually a pair of crank mechanisms including a pair of connecting rods connected to said pair of common crankshafts. On the other hand the double-acting pump cylinder - piston assembly has a pair of driving mechanisms including a pair of O-members engaged with the crank pins of said pair of common crankshafts, so that the two two-stroke cycle power cylinder - piston assemblies and the double-acting pump cylinder - piston assembly are operated in synchronization with each other. In this formerly proposed engine, the crank radius of each of the said pair of common crankshafts with respect to the power cylinder - piston assemblies was substantially the same as that with respect to the pump cylinder - piston assembly, so that the strokes of the power pistons of the power cylinder - piston assemblies were substantially the same as the strokes of the pump pistons of the pump cylinder - piston assembly.
However, up to the present date, the most desirable mechanism which can change high speed rotary motion, as in engines, to corresponding high speed reciprocating motion most definitely, without any substantial play, vibration, or failure, is a crank mechanism composed of a crankshaft and a connecting rod. Therefore, in the aforementioned formerly proposed two-stroke cycle gasoline engine having a double-acting pump cylinder - piston assembly incorporating two horizontally opposed pistons, it is, of course, desirable that the pump pistons should be connected with said pair of crankshafts by a pair of crank mechanisms each including a connecting rod, if possible. However, in the case of a double-acting pump cylinder - piston assembly in which the smaller end portion of a connecting rod, i.e. the end of a connecting rod opposite to its larger end where the connecting rod engages with a crank pin, cannot be directly connected with a pump piston, but must be connected with the outer end of a push rod which extends through an end plate and is connected with a pump piston at its inner end, since a connecting rod is exerted with a side force which acts in a direction perpendicular to the direction of reciprocation of a piston, a cross head is required at the connecting portion of the push rod and the connecting rod so as to support the side force. However, since the aforementioned two-stroke cycle gasoline engine is particularly intended for use as an engine for a small-size automobile, it is severely limited with regard to its width due to the limited space available in the engine compartment of a small-size automobile, and therefore in the structure of the aforementioned formerly proposed engine, wherein the pump piston of the pump cylinder - piston assembly has substantially the same piston stroke as the power piston of the power cylinder - piston assembly, it is absolutely impossible to obtain enough space for providing the aforementioned cross head. The same problem is recognized with respect to the two-stroke cycle diesel engine which we have proposed in co-pending U.S. patent application Ser. No. 966,597 now U.S. Pat. No. 4,248,183.